1. Field of the Invention
The present invention relates to a method for producing a polyester useful for various applications, comprising further polymerizing a polymeric saturated polyester with the terminal group being substantially a hydroxyl group.
2. Description of the Related Art
It has been well known that saturated polyesters, in particular those containing terephthalate as one component, have been utilized as film, molded articles and glass fiber reinforced plastics for applications in various fields.
They are also useful as base resins for adhesives and paints, so their development has also been progressing recently.
The present invention relates to a method for imparting satisfactory practical properties to polyesters such as those with relatively low melting points to be used as paints and adhesives and aliphatic polyesters not sufficiently provided with film forming properties, etc. by increasing the molecular weights thereof rather than polyesters with relatively high melting points such as polyethylene terephthalate.
As has been well known, the method for synthesizing a polyester of a high molecular weight (the term "high-molecular weight" means a number average molecular weight of about 10,000 or more) depends on the deglycol reaction of the terminal hydroxyl group of a low-molecular polyester.
Hence, the concentration of the terminal group decreases remarkably as the molecular weight increases. Furthermore, a decomposition reaction additionally occurs due to the temperature during the ester exchange, so a limitation is imposed on the molecular weight.
Such a tendency can be distinctively observed in aliphatic polyesters in particular. When a conventional deglycol reaction in reduced pressure is employed for producing a high-molecular saturated polyester as is shown in FIG. 1, for example, it is observed that the molecular weight begins to decrease once the molecular weight reaches the maximum.
In such case, it is very hard to produce an aliphatic polyester with a molecular weight satisfactory for forming tough film by means of the conventional deglycol reaction. In other words, it may be concluded that a film with mechanical properties for practical use cannot be formed from the molecular weights which have been realized by aliphatic polyesters.
Polyesters with aromatic structures also induce similar situations, for example, when the polyesters are to be used as a base resin of an adhesive. The unsatisfactory molecular weights thereof appear as insufficient mechanical properties.